For the coating of running webs, particularly of paper or cardboard, devices having a backing cylinder guiding the web are known where an application system with a dosing element for setting the desired coating weight is mounted in the area where the cylinder is wrapped by the web. The surface of the dosing element extending over the working width and the cylinder together create a gap converging in the direction of the web travel, through which the coating material is guided towards the web. The desired coating weight can be set by control of the gap width at the narrowest point of the application gap, which is called the dosing or metering line. In order to improve the coating quality and/or to reduce the coating weight (applied amount of coating material per square meter of web surface), it is known to provide an additional dosing or metering system beyond the application system, which wipes off the excess of the predosed applied coating, e.g. with a wiper blade, bringing it to the desired coating weight.
DE-OS 39 16 620 describes a coating cylinder dipping into a liquid-containing chamber and forming an application gap with the cylinder. A nonelastically supported predosing unit, reaching into the area of the application gap and closing it to define a chamber on the outgoing side, forms a predosing gap together with the cylinder, whereby an outflow channel is left free towards the coating cylinder.
DE-OS 36 20 374 describes a device without an immersed coating cylinder, wherein the coating material is pumped through a channel into the converging gap between the cylinder and the wiper element. Rotating wiper bars or distributor elements with a curved cross section, which are elastically supported and can be pressed against the cylinder are provided as wiper elements, in order to set the line pressure at the dosing line.
In the coating of absorbent paper or cardboard webs the coating material--usually a coating dye in an aqueous solution--has to be fed to the web with as low a pressure as is possible, in order to prevent an undesired penetration into the web. It must be ensured that the coating dye is evenly distributed and that uncoated portions are avoided. If after the application an additional final dosing system is effected, the removal of the excess also must be as even and as small as possible. In practice it has proven difficult to achieve this with coating dyes of various compositions and at ever increasing web speeds without undesired side effects, such as film splitting, high wear, etc.